The present invention relates to an optical pickup apparatus by which a light flux is converged onto an information recording surface of an optical information recording medium, and to an optical element (lens) used for the optical pickup apparatus.
There have been developed optical pickup devices which are used for recording data on or reproducing data from optical disks (optical information recording media) such as MO, CD and DVD, and they are used for various purposes.
In these optical pickup devices, there is used a light flux (laser light flux) whose wave front is trued up uniformly.
In the case of an optical disk of a photomagnetic recording type including MO, an optical disk of an organic dye recording type including DVD-R, and of an optical disk of a phase change recording type including DVD-RW, energy density (power) of a laser beam is enhanced by increasing an amount of electric current to be supplied to a laser oscillator, when writing data, and energy density of a laser beam is lowered by decreasing an amount of electric current to be supplied to a laser oscillator, when reading data.
For example, when writing data in MO, a magnetic layer of MO is irradiated with a laser beam until the temperature of the magnetic layer is raised up to Curie point of 150° C.–300° C. so that the condition under which the data can be rewritten may be created. If the magnetic force is given to this region by a bias magnet, data can be rewritten.
In the operations for writing MO, erasing operations, writing operations and verifying operations are carried out within a period of time in which the disk makes three turns.
In the first turn of the disk, the bias magnet is brought near to the reverse side of the MO disk that is rotating at high speed and, at the same time, the sector on which the data are written is irradiated by a strong laser light flux to raise temperature so that the direction of magnetization may be uniformed. This is the erasing operation which means that data of zero are written entirely.
In the second turn of the disk, the direction of the bias magnet is reversed, and a strong laser light flux is irradiated intermittently only on the location where data of 1 are written to raise temperature so that the direction of magnetization may be changed.
Then, in the third turn of the disk, a weak laser light flux is irradiated on the magnetic layer, and its reflected light is checked in term of an inclination angle, and thereby, written data are read to confirm whether the data are written correctly or not.
When writing data in DVD-RW, a strong laser light flux is irradiated on the recording layer of DVD-RW, and when temperature of the recording layer is raised up to 400° C., there is created the state of crystal wherein molecules on the recording layer are arranged in the same direction. Further, if the temperature is raised up to 600° C. or higher by irradiating stronger laser light flux, the recording layer is melted, and molecules are scattered in all directions, and after that, when the disk rotates and the laser light flux stops irradiating, resulting in the fall of temperature, the recording layer is fixed to be in the amorphous state. Since the reflectance for light on the section in the crystallized state is different from that on the section in the amorphous state, written data are read by investigating the intensity of the reflected light by irradiating the disk with a weak laser light flux.
On the optical disk on which data can be written as stated above, such as MO and DVD-RW, data reading and data writing are repeated alternatively. At the moment when the state of reading is switched to the state of writing, the power of the laser light flux emitted from the laser oscillator rises, causing a wavelength of the laser light flux to become long instantaneously (“mode hop”).
When the wavelength of the laser light flux is long, a position of the light-converged light spot formed on the optical axis is moved, by dispersion property in a material of lens, to be far from an objective lens (“axial chromatic aberration”). Namely, the position of the light-converged light spot is deviated from the recording surface of the optical disk, causing a fear that errors may occur when information is written on the optical disk.
A wavelength of a laser light flux used for MO is 600–700 nm, and a variation of the wavelength caused by mode hop is about a few tenth nm. It is necessary to correct axial chromatic aberration on the wavelength variation of this level.
A high density DVD (HD-DVD) having recording density that is higher than that in DVD available on the market and an optical pickup device for the high density DVD are developed presently. A wavelength of a laser light flux used for HD-DVD is 400–500 nm, and a variation of the wavelength caused by mode hop is about a few nm. Since a wavelength of a laser light flux used for HD-DVD is shorter than that of a laser light flux used for MO, the axial chromatic aberration turns out to be great. Therefore, the necessity for correcting the axial chromatic aberration is higher than that for correcting MO.